The principles of acoustic levitation are well known and are disclosed in various references. Generally, intense sound forces are employed to position or support an object. Some levitators utilize standing waves in a tuned cavity, as described in U.S. Pat. No. 3,882,732. Others employ interference from a reflector or from an opposed sound source, as described in U.S. Pat. No. 4,284,403. In all cases, reflected sound waves form zones of minimum pressure or energy wells in which objects may be levitated.
A major potential application for acoustic levitation is containerless melting. An object is levitated by acoustic forces and may be heated and cooled without the possibility of being contaminated by contact with a container. Several proposals have been made for the processing of acoustically positioned objects in future space stations.
To exert the forces necessary to levitate or position dense objects, it is desirable to produce coherent and intense sound in the region of the suspended object. In the past, the best available sources have been solid pistons, as described in U.S. Pat. No. 4,284,403. Such sound sources, however, are highly inefficient because of the poor impedance match between the piston and air. As a result, the use of large amounts of power are required to produce the required sound pressure levels.
The use of a furnace with acoustic levitation also poses several additional problems which have not been adequately resolved. The piston or sound source must be coupled to the furnace cavity and therefore is subjected to considerable heat flux, which may damage the source. Also, since the piston is cooler than the furnace, a temperature gradient is established between the source in the furnace. As the sound moves through the gas in the temperature gradient, the wavelength increases relative to the size of the source, causing increased dispersion of the sound and loss of levitating forces in the furnace. These problems become particularly severe at higher operating temperature, i.e., above 1500.degree. C., since the amount of heat power radiated per unit area closely follows the fourth power temperature law.
Moreover, prior art acoustic levitation devices have only been considered practical for use in space or in an environment free of gravitational forces. It would be desirable to provide such a device which could be practically employed to levitate dense objects on earth, or against the force of gravity.
In view of the foregoing, it would be desirable to provide a sound source that could be used in a high temperature environment without excessive exposure to heat and without excessive dispersion of sound across a temperature gradient. Also, in general, it would be desirable to provide a more efficient sound source to increase the forces available for acoustic levitation.